This invention relates to generally to the field of greenhouse construction, and more particularly to an insulated greenhouse.
Greenhouses (also known as glasshouses, hothouses, coldframes, and the like) are frame structures (made of wood, metal or other materials) that are covered with a transparent layer or layers (generally plastic film, rigid plastic, or glass), which permit natural light to enter, while at the same time maintaining a barrier from the elements. Greenhouses work on the principle that shorter wavelength light passes through the transparent layers to be absorbed within the structure, driving photsynthesis and providing heat. The envelope of the greenhouse provides some resistance against loss of heat, and the interior is kept warm partly through absorbed solar energy, and partly by the addition of auxiliary heat.
Presently, greenhouses are generally built without insulation, and this creates a number of problems, including high heating costs, condensation problems, temperature variation throughout the greenhouse, and limited ability to store daytime heat. It would be desirable to add insulation, but conventional insulation has not proved applicable to greenhouses because it blocks light transmittance.
An object of the invention is to alleviate this problem.
According to the present invention there is provided a greenhouse comprising a frame structure, an outer transparent covering layer on said frame structure, a transparent inner layer, and a transparent insulating sheet-like material between said inner and outer layers in close proximity to one of said layers and spaced from the other of said layers, said insulating sheet-like material having a cellular structure with the axis of its cells generally oriented normal to the plane thereof so as to transmit incident light therethrough.
This insulating material provides good heat insulation without significantly affecting light transmission. It insulates by suppressing convection through its small cell size to create a significant thickness of xe2x80x98dead airxe2x80x99, and also provides infrared blocking properties to the extent that the cell walls absorb infrared radiation.
The transparent insulation should be normally spaced from the outer transparent covering so as to define a gap, typically an air gap, and be in intimate contact with the transparent inner layer, which then closes the cells to prevent air circulation and also act as a warm vapor barrier. The inner transparent layer preferably also serves as a support for the transparent insulation.
This support can be an underlying glazing layer. The inner glazing, however, is not required to bear a significant load, such as wind, snow and ice. This can be used to gain two important advantages, namely:
i) maximization of light transmittance The addition of a second glazing leads to some loss of light transmittance because of the less-than-unity transmittance of the glazing itself, as well as the loss-of-aperture caused by introduction of opaque framing elements. However, because this glazing is not required to bear any major loads (e.g. wind and snow), the supporting frame can small and widely spaced to minimize aperture loss; there is much more freedom to choose the inner glazing material on the basis of its light transmittance (for example, a thin layer of very-high transmittance PTFE film can be used, even though a film thick enough to be used in load-bearing situations would be prohibitively expensive); and the inner glazing layer and the transparent insulation which is in contact with the inner glazing layer can be oriented so that the sun is incident at an optimal angle for minimum loss by reflection or scattering.
ii) the inner glazing can be optimized to minimize cost, compatible with standard greenhouse systems for natural ventilation, and/or adapted to optimize condensation runoff.
This invention makes use of the fact that honeycomb transparent insulations need only be in close contact with one glazing to be effective, and that gravity, along with tension and curvature, can be used to keep transparent insulation in close contact with an underlying glazing. Thus an inner glazing can be installed in a greenhouse, and the transparent insulation simply laid on top of it. In particular, the transparent insulation should be mounted in a manner that leaves a significant air gap between the transparent insulation and the outer glazing. This air gap can be used for a number of purposes, including i) prevention of crushing of transparent insulation by external loads, ii) inflation of outer glazing, iii) removal of moisture by ventilation or dehumidification, iv) controlled heating to defrost, de-ice or remove snow from, the outer glazing. In the absence such an air gap, moisture build would destroy the optical properties, lead to the growth of mildew and algae, and destroy the insulation.
In particular, this invention also teaches how to attach the transparent insulation to greenhouses in such a way that it stays in intimate contact with at least one glazing, as well as how to solve problems of crushing, control and removal of moisture from the insulation cavity, inflation to remove slack from outer film glazings, and removal of snow, ice, and frost from the exterior of the greenhouse.
Honeycomb transparent insulation material refers to a class of materials that are designed to transmit a maximum amount of light incident upon them, while providing a significant amount of insulation value. They have a cellular structure, with the axis of the cells being oriented through the structure, in such a way that i) dead air spaces are created; ii) thermal infrared radiation is interfered with to some degree; and iii) incident light is either transmitted or forward reflected by the cell walls. The concept of honeycomb transparent insulation was first suggested by Dr. K. G. T. Hollands while a graduate student at McGill University, Montreal, Canada, and has been extensively researched and a number of publications exist on the subject. Several commercially-available transparent insulations are available. These include rigid materials made by extrusion, or bundling of capillaries, and also a new generation of film-based materials that are semi-rigid. Examples are Okalux Kappilarglas Gmbh. of Marktheidenfeld-Atfeld Germany, which makes kappilux(copyright) capillary-bundled transparent insulation, Arel Energy Ltd. of Yavne, Israel, which makes Arel(copyright) extruded transparent insulation, and of course Advanced Glazings Ltd., North Sydney, NS Canada which makes InsolCore(copyright) film-based transparent insulation.
By using this invention, a greenhouse structure can be built or retrofitted in a way that the benefits of an insulated building envelope are obtained, while retaining high transparency and the benefits of natural light (i.e. a structure can be built that has lower heating costs, less condensation, and less variation of temperature throughout the internal volume, than normal uninsulated greenhouse construction, yet retains the ability to admit natural light to provide for plant growth).
The invention also solves a number of practical problems that occur when applying transparent insulation to greenhouses:
1) this invention provides a practical and economical technique for applying the material. To date, lack of a practical attachment method has blocked the use of transparent insulation in greenhouses.
2) this invention keeps the honeycomb transparent insulation in close proximity to at least one glazing, by using gravity, and possibly tension with curvature. This is a necessary condition that must be met in order for the honeycomb transparent insulation to block convection and provide insulation value.
3) The invention may provide an air gap between the transparent insulation and the outer glazing. This solves the following problems:
i) without the air gap, the transparent insulation could be crushed and damaged if the outer glazing deflected downwards, as it is likely to do under snow and wind load. This is important with rigid outer glazings, and critical with semi-rigid or film-type outer glazings.
ii) buildup of water and condensation in the transparent insulation can be prevented, by any combination of dehumidification, circulation, and external venting of this air gap.
iii) snow, frost, and ice build-up on the outer surface of a greenhouse can cause loss of light transmittance, and excessive physical loading of the glazing and structure. With traditional poorly-insulating glazings, snow, frost, and ice buildup can be controlled by simply turning up the heat in the structure. In greenhouses covered with an inflated double polyethylene film, the source of inflation pressure is turned off, which lets the outer layer collapse onto the inner layer, thus negating the insulation value of the air gap between the films and allowing internal greenhouse heat to melt snow on the external surface of the greenhouse. On the other hand, the use of honeycomb transparent insulation could potentially increase problems of snow buildup, because it slows the transfer of internal heat from the inside of the greenhouse to the external surface, and thus slows the rate of melting. But the use of this invention to attach the transparent insulation allows the problem of snow buildup to be addressed by mounting a heat source in the air gap above the transparent insulation and below the outer glazing. In this way, heat can be applied to defrost or melt snow on the outer glazing, thus solving the snow/ice/frost problem.
iv) if a plastic film is used as an outer glazing, it is sometimes desirable to use air pressure to inflate the outer glazing so that the film is under constant tension. This prevents violent flapping or oscillatory movement of the plastic due to wind action, which can rapidly destroy the film. In standard uninsulated film greenhouse construction, this is done by introducing pressurized air into the gap between two poly films which are both mounted on the outside of the greenhouse framework. But such conventional construction is not suitable for mounting transparent insulation because the transparent insulation would get crushed between the layers. However, this invention allows the use of a film as an outer glazing mounted on the outside of the greenhouse frame, in addition to the lower glazing (which may be film or rigid) which supports the transparent insulation, and one which is mounted in on the outside of the main roof frame. And if care is taken to ensure that the air gap between the inner and outer glazings is sealed, pressurized air may be introduced into this air gap, thus inflating the outer glazing and preventing slack-tension related wind damage to this outer glazing.
The invention also provides a method of insulating a greenhouse comprising the steps of placing transparent insulating sheet-like material between an outer transparent covering layer and a transparent inner layer, said insulating sheet-like material having a cellular structure with the axis of its cells generally oriented normal to the plane thereof so as to transmit incident light therethrough and being in close proximity to one of said layers.
This invention describes a practical system for mounting transparent insulation in the roof of sidewall greenhouses, or in tunnel-type greenhouses.